The use of hydrostatic pressure control at the nozzles of ink jet printheads has been utilized to effectively maintain an ink meniscus in each nozzle. The importance of maintaining such meniscus is to ensure that, after the printhead has been energized to eject drops of ink, a column of ink remains between the ink channels within the printhead and the printhead nozzles. If such column of ink is not maintained, air will enter the ink delivery system negatively impacting the ability of the printhead to print. If the ink reservoir were simply positioned above the nozzles, a column of ink would be maintained, but the positive hydrostatic pressure created at the nozzles would cause the ink to drip out. Accordingly, ink delivery systems which maintain a negative pressure at the printhead nozzles have been developed. The negative pressure on the supply side of the nozzles, if properly regulated within predetermined parameters, is sufficient to ensure that an ink meniscus which forms in the nozzles remains in place taking into account the viscosity and specific gravity of the particular ink.
One way in which such negative pressure is maintained is through the use of ink jet cartridges having a foam insert in the ink reservoir. This type of ink supply device can be mounted above the level of the printhead nozzles because a negative pressure at the nozzles is maintained the capillary action of the ink in the foam. However, this type of system is typically used in small ink volume situations.
Alternatively, it is known to position the ink reservoir below the vertical level of the printhead nozzles to maintain the desired negative pressure at the printhead nozzles. Basic hydrostatic pressure principles apply such that the relative position of the free surface of ink in the ink reservoir to the nozzles determines the resulting negative hydrostatic pressure at the nozzles. In these systems when the nozzles are energized to eject ink, additional ink is drawn from the reservoir by capillary action into the ink channels of the printhead. After energization of the printhead has ended, the controlling of the negative hydrostatic pressure at the nozzles correspondingly controls the meniscus position at the nozzles to ensure proper operation of the printhead.
It is also known to use ink jet printing systems in mail handling systems such as the postage cancellation device described in United States Application entitled "POSTAL CANCELLATION MACHINE" and filed on Oct. 03, 1997 under Attorney Docket No. E-663. In the aforementioned postage cancellation machine, multiple printheads are needed in order to print various size cancellation marks on different sized mailpieces being processed through the cancellation machine. Moreover, since the mailpieces are transported on edge and the various size cancellation marks differ in height along the vertical dimension of the transported mailpieces, each of the plurality of printheads are offset from each other in the vertical direction in order to selectively energize the printheads for printing large and small cancellation marks. However, since the operating parameters of each printhead are the same, the negative pressure at each printhead must be maintained within a desired range taking into account the difference in hydrostatic pressure resulting from the different printhead elevations. The hydrostatic pressure control principles discussed above can be used to maintain the desired negative pressure by providing each printhead with its own ink delivery system. However, the use of multiple independent ink delivery systems drives up the cost of the product and lowers its expected reliability.